.\" Copyright (c) 2008-2012 Apple Inc. All rights reserved.
.Dd May 1, 2009
.Dt dispatch_group_create 3
.Os Darwin
.Sh NAME
.Nm dispatch_group_create ,
.Nm dispatch_group_async ,
.Nm dispatch_group_wait ,
.Nm dispatch_group_notify
.Nd group blocks submitted to queues
.Sh SYNOPSIS
.Fd #include <dispatch/dispatch.h>
.Ft dispatch_group_t
.Fo dispatch_group_create
.Fa void
.Fc
.Ft void
.Fo dispatch_group_enter
.Fa "dispatch_group_t group"
.Fc
.Ft void
.Fo dispatch_group_leave
.Fa "dispatch_group_t group"
.Fc
.Ft long
.Fo dispatch_group_wait
.Fa "dispatch_group_t group" "dispatch_time_t timeout"
.Fc
.Ft void
.Fo dispatch_group_notify
.Fa "dispatch_group_t group" "dispatch_queue_t queue" "void (^block)(void)"
.Fc
.Ft void
.Fo dispatch_group_notify_f
.Fa "dispatch_group_t group" "dispatch_queue_t queue" "void *context" "void (*function)(void *)"
.Fc
.Ft void
.Fo dispatch_group_async
.Fa "dispatch_group_t group" "dispatch_queue_t queue" "void (^block)(void)"
.Fc
.Ft void
.Fo dispatch_group_async_f
.Fa "dispatch_group_t group" "dispatch_queue_t queue" "void *context" "void (*function)(void *)"
.Fc
.Sh DESCRIPTION
A dispatch group is an association of one or more blocks submitted to dispatch
queues for asynchronous invocation.
Applications may use dispatch groups to
wait for the completion of blocks associated with the group.
.Pp
The
.Fn dispatch_group_create
function returns a new and empty dispatch group.
.Pp
The
.Fn dispatch_group_enter
and
.Fn dispatch_group_leave
functions update the number of blocks running within a group.
.Pp
The
.Fn dispatch_group_wait
function waits until all blocks associated with the
.Fa group
have completed, or until the specified
.Fa timeout
has elapsed.
If the
.Fa group
becomes empty within the specified amount of time, the function will return zero
indicating success.
Otherwise, a non-zero return code will be returned.
When
.Va DISPATCH_TIME_FOREVER
is passed as the
.Fa timeout ,
calls to this function will wait an unlimited amount of time until the group
becomes empty and the return value is always zero.
.Pp
The
.Fn dispatch_group_notify
function provides asynchronous notification of the completion of the blocks
associated with the
.Fa group
by submitting the
.Fa block
to the specified
.Fa queue
once all blocks associated with the
.Fa group
have completed.
The system holds a reference to the dispatch group while an asynchronous
notification is pending, therefore it is valid to release the
.Fa group
after setting a notification block.
The group will be empty at the time the notification block is submitted to the
target queue.
The group may either be released with
.Fn dispatch_release
or reused for additional operations.
.Pp
The
.Fn dispatch_group_async
convenience function behaves like so:
.Bd -literal
void
dispatch_group_async(dispatch_group_t group, dispatch_queue_t queue, dispatch_block_t block)
{
	dispatch_retain(group);
	dispatch_group_enter(group);
	dispatch_async(queue, ^{
		block();
		dispatch_group_leave(group);
		dispatch_release(group);
	});
}
.Ed
.Sh RETURN VALUE
The
.Fn dispatch_group_create
function returns NULL on failure and non-NULL on success.
.Pp
The
.Fn dispatch_group_wait
function returns zero upon success and non-zero after the timeout expires.
If the timeout is
.Va DISPATCH_TIME_FOREVER ,
then
.Fn dispatch_group_wait
waits forever and always returns zero.
.Sh MEMORY MODEL
Dispatch groups are retained and released via calls to
.Fn dispatch_retain
and
.Fn dispatch_release .
.Sh FUNDAMENTALS
The
.Fn dispatch_group_async
and
.Fn dispatch_group_notify
functions are wrappers around
.Fn dispatch_group_async_f
and
.Fn dispatch_group_notify_f
respectively.
.Sh SEE ALSO
.Xr dispatch 3 ,
.Xr dispatch_async 3 ,
.Xr dispatch_object 3 ,
.Xr dispatch_queue_create 3 ,
.Xr dispatch_semaphore_create 3 ,
.Xr dispatch_time 3
.Sh CAVEATS
In order to ensure deterministic behavior, it is recommended to call
.Fn dispatch_group_wait
only once all blocks have been submitted to the group.
If it is later determined that new blocks should be run, it is recommended not
to reuse an already-running group, but to create a new group.
.Pp
.Fn dispatch_group_wait
returns as soon as there are exactly zero
.Em enqueued or running
blocks associated with a group (more precisely, as soon as every
.Fn dispatch_group_enter
call has been balanced by a
.Fn dispatch_group_leave
call).
If one thread waits for a group while another thread submits new blocks to the
group, then the count of associated blocks might momentarily reach zero before
all blocks have been submitted.
If this happens,
.Fn dispatch_group_wait
will return too early: some blocks associated with the group have finished,
but some have not yet been submitted or run.
.Pp
However, as a special case, a block associated with a group may submit new
blocks associated with its own group.
In this case, the behavior is deterministic: a waiting thread will
.Em not
wake up until the newly submitted blocks have also finished.
.Pp
All of the foregoing also applies to
.Fn dispath_group_notify
as well, with "block to be submitted" substituted for "waiting thread".
